An optical waveguide router is an optical device that is supplied with one or more optical signals, typically of different wavelengths, at one or more input ports, and provides these signals at one or more output ports.
As shown in FIG. 1, such a router 100 typically includes an input free-space slab 110, such as a star coupler with one or more input ports 112A, 112B connected thereto, and an output free-space slab 120, also such as a star coupler, with one or more output ports 122A, 122B. An optical grating 130 comprising a succession of optical waveguides 132A, 132B, 132C of different optical lengths, is connected between the input and output free-space slabs. Typically the grating includes at least ten, usually at least twenty and often a hundred or more waveguides. The number is usually dependent on the number of channels, each of a different wavelength, to be routed by the router, in a wavelength-division multiplexed transmission system.
It is known that transmission characteristics of such a router have a temperature dependence, largely as a result of the effect of temperature changes on the lengths and refractive index of the arms of the grating.
My earlier U.S. Pat. No. 5,629,991 that issued on May 13, 1997 relates to techniques for reducing birefringence effects in an optical waveguide router. Birefringence in such routers arises primarily because of the difference in strains in the different materials of the router that are introduced, typically during manufacture. In an optical waveguide of the kind involved, a silica layer which forms the waveguide is supported on a silicon substrate. As a result of the large difference in this thermal expansion coefficients of silica and silicon, large compressive strains are produced in the silica layer as a result of heating steps performed in the fabrication process. The resulting birefringence caused by the strains produces different propagation constants for the TE and TM waveguide modes. Because these two modes have different propagation constants, an optical signal propagating in the router is split into two components corresponding to the TE and TM modes. These components are characterized by different wavelengths of maximum transmission and there is a resulting polarization shift that can reduce efficiency and also increase cross talk between any two channels that are closely spaced in wavelength.
In this prior patent, the polarization shift is reduced by the step of imparting a prescribed curvature to the plane of the optical grating along a line that traverses the successive waveguide arms in a direction substantially perpendicular to that in which the optical signal propagates. The curvature preferably is imparted by flexing the optical grating at two points respectively located near the longest and shortest of the waveguide arms of the grating. The flexing is obtained by a pair of planar elements spaced apart on a common support between which the grating extends.